In a conventional method for fabricating a semiconductor device, an electrically conducting hole, i.e., a contact needs to be provided at a predetermined position. As a conductive material, aluminum(Al) which is supplied by sputtering has been used for a long time. However, along with recent high-integration of semiconductor device, a diameter of contact reaches a level of sub-micron and the aspect ratio(a ratio of height and width) increases, thereby deteriorating a step coverage in the aluminum contact and causing a short circuit.
To solve this problem, a method has been developed in which tungsten is buried into a contact before sputtering. For example, tungsten film is first formed on a semiconductor substrate by using the CVD method, thereafter etching back the entire surface of the tungsten film by dry etching. At this time, the tungsten film on the surface of the substrate can be removed, but tungsten buried into the contact remains. Then, aluminum is sputtered to contact the tungsten in the contact, thereby forming an electric conduction therebetween. In this method, the process of tungsten CVD will be explained referring to FIG. 1.
FIG. 1 is a schematic diagram showing a conventional tungsten CVD device. As shown, in a reaction room 1 which is made of metal, a cathode 2 and an anode 3 are disposed opposite to each other, where the bottom part of the cathode 2 has many small holes that function as inlets leading gas. Furthermore, the cathode 2 is connected to a high-frequency power source 4, and the anode 3 also functions as a ground. To the exhaust of the reaction room 1, a removal device 7 as to nitrogen trifluoride(NF3) is connected through a pump 6.
In operation, a semiconductor substrate 5 is disposed on the anode 3, then flowing a gas such as tungsten hexafluoride(WF6) of 200 to 400 sccm from the gas inlets the into the reaction room 1 while generating a high frequency of 13.56 MHz at 100 to 300 W from the high-frequency power source 4. Then, by adjusting the inner pressure of the reaction room to be about 0.5 to 0.7 Torr by controlling the amount of exhaust gas and raising the temperature of the anode 3 at 300.degree. to 500.degree. C., tungsten film can be formed on the semiconductor substrate 5.
The tungsten film is also formed on the inside wall of the reaction room 1, the surface of the cathode 2 and the surface of the anode 3 except the surface where the semiconductor substrate 5 is disposed, and the deposition thickness is gradually increased with the progress of deposition. Since the deposition film of tungsten on the inside wall of the reaction room 1 etc. causes particles released from there as the film thickness is increased, they should be removed.
This removal is conducted after the tungsten film is formed on the semiconductor substrate 5 and the semiconductor substrate 5 is taken out from the reaction room 1. After taking out the substrate 5, the reaction room 1 is vacuumed to exhaust the remainder of WF6 gas used in the deposition of tungsten film, thereafter leading a fluoride gas such as nitrogen trifluoride(NF3) of 100 to 200 sccm into the reaction room 1, then generating a high frequency of, for example, 13.56 MHz at 300 to 500 W while holding the degree of vacuum of the reaction room 1 to be, for example, 100 to 150 mm Torr.
The time required in this removal treatment is determined by the thickness of the tungsten film attached to the inside of the reaction room 1. For example, if the deposition of 500 nm film is once conducted, the removal treatment on the above condition takes about 2 min. and 30 sec. Such etching treatment can be conducted after the tungsten CVD of the semiconductor substrate 5, thereby removing the tungsten film attached to the inside of the reaction room 1 etc. to prevent the particles from releasing. The etching reaction is progressed as shown by: EQU W+2NF3.fwdarw.WF6+N2 (1)
On the other hand, the NF3 remainder gas which is not reacted with W is exhausted through the pump 6 into the removal device 7, where it is treated to exhaust only nitrogen(N2). The removal device 7 contains a material for adsorbing fluorine which is changed once about three to six months. Thus, the NF3 gas which is harmful can be prevented from directly exhausting into the air. WF6 obtained by the above reaction (1) is diluted by a sufficient N2, thereafter released into the air, therefore causing no problem in safety.
However, since NF3 used in the plasma etching to the reaction room 1 is an earth-warming-substance, it may be prohibited in the future. Furthermore, there is a problem that the removal device for treating the harmful NF3 gas is costly.